[RocketsNW] Rockets Digest, Vol 15, Issue 21

[Paul Bogdanich]

The formula I was given by Wedge was 1 gram of thermite per 1000 N-s of motor.

We had a chat about this somewhere, we figured core volume would be a 
better way to determine amount.


The way you figure it is for APCP it takes between 2 and 10 cal / cm2 / sec 
to get it to light depending on the particular fuel.  So for rule of thumb 
purposes if you figure 10 cal / cm2 / sec your safe.  For CuO Al thermite 
the heat or reaction is about .92 kcal/gram as I recall but the question 
with thermites is how many cm2 are you heating.  Since thermites are 
generally low gas, for CuO Al thermite the answer is not many.  In effect 
the thermite is heating one grain, two at the most, at the top of the motor 
and the exhaust from that grain is catching the rest of the motor.  This 
partial ignition method is by far the easiest and works pretty good but to 
light a single grain you don't need anything near a full gram of material 
for a 1,000 NS motor.  Also think about scaling that up.  20 grams of 
material for a N motor?  With as much room as you have to work with in an N 
motor a basket burning fuel pellets would work a lot better.  The trick 
though is the small motors where you don't have room for a basket and want 
full ignition.  So as it happens 38s are harder to light via full ignition 
than 98s are.  Of course a 38 is easy to light just not all at 
once.  That's the trick.